Insulation for electrical conductors



March 1964 A. R. ANDERSSON ETAL 3,127,470

INSULATION FOR ELECTRICAL CONDUCTORS Original Filed Feb. 19, 1960 Fig/Wrapping Me 001/ MM 0 perfinrafeo insu/ming' mmeria/ /mpreg'7a/irg #:ewrapped goi/ whh a fluid resinous impregnbm Fans/erring #be impregnan1/o #:a solid s/a/e United States Patent 3,127,470 INSULATION FORELECTRICAL CONDUCTORS Anders Ragnar Andersson and Knut Nylund, Vasteras,Sweden, assignors to Allmiinna Svenska Elektriska Aktiebolaget,Vasteras, Sweden, a corporation of Sweden Continuation of applicationSer. No. 9,926, Feb. 19, 1960. This application Feb. 26, 1963, Ser. No.261,179

Claims priority, application Sweden Feb. 24, 1959 3 Claims. (Cl.174-121) The present invention relates to an insulation for electricalconductors, especially for high voltage conductors.

This application is a continuation application of our copendingapplication Serial No. 9,926, filed February 19, 1960, entitledInsulation for Electrical Conductors, now abandoned.

It entails great difficulties to reach a complete filling of theinsulation of a conductor, which consists of several layers of a tape orsheet formed insulation material Wrapped around the conductor, byimpregnation with a plastic material. Problems occurring will be furtherdealt with in conjunction with the following description of themanufacture of a group of products, high voltage coils being chosen asan example.

The manufacture of high voltage coils is often carried out in such a waythat mica tape or mica sheet consisting of mica flakes which overlapeach other and are attached to a tape or sheet formed backing, e.g.,paper or cloth, is wrapped in several layers around bundles ofconductors composed of individual conductors, after which the wrappedbundles of conductors are subjected to a further treatment usuallyconsisting of an impregnation with an impregnant and a subsequentmoulding of the insulation. Before this impregnation moisture and othervolatile components are usually removed. The impregnation is made inorder to fill voids and air spaces occurring in the insulation, and inthis way, give the insulation good electrical and mechanical properties.To ensure that the filling is good, the impregnation is often made by avacuum-pressure process and by using as an impregnant asphalt or a fluidsolventless plastic material. Incomplete filling leads to internaldischarges in the remaining voids of the insulation when the insulationis subjected to electric stresses. The discharges destroy the insulatingmaterials and as a result, the insulation as well, at stresses which areessentially lower than that which the insulation should withstand if novoids existed. Punctures and other electrical faults thus occur after aconsiderably shorter time and at lower stresses in an incompletelyfilled insulation than in a completely filled one.

In spite of the use of vacuum-pressure during the impregnation, it hasproved difficult to get a satisfactory penetration of the impregnant andso a good filling of voids occurring in the insulation. In order tobring forth a good penetration and filling, diifer'ent steps have beensuggested. Thus, it is known to use a material soluble in the impregnantas binder for the mica tapes and by this means make the penetration ofthe impregnant easier than if the binder was not influenced by theimpregnant. The use of mica tapes in which the binder is volatile or canbe volatilized by heat treatment, and so can be removed before theimpregnation, is also known. Further, the use of mica tapes, in whichthe mica flakes are attached to the backing at various points has beensuggested. In the processes described above, attention has thus beendirected towards avoiding the use of binders in the mica tape, whichbinders hinder the penetration of the impregnants, through a suitablechoice of binder or of method of attaching the mica flakes to thebacking. It has also been suggested to use backings soluble in theirnpregnant to reach the intended effect.

Though binders and backings for the tapes in the described insulationshave been adapted with the intention of facilitating the penetration ofthe impregnant, all insulations, however, have the disadvantage from thepoint of view of impregnation that they contain tapes or sheets whichhave layers of impervious mica flakes overlapping each other. Theselayers constitute, irrespective of the binders and backings which areintegral parts of the tapes or sheets, a very great obstacle to thepenetration of the impregnant, especially in the direction perpendicularto the plane of the tapes or sheets.

The same disadvantages from the point of view of impregnation are foundwith insulations which are built up of several layers of self-supportingor self-carrying mica sheets consisting of diminutive mica flakes whichoverlap each other and which are held together by molecular forcesacting between the individual flakes. Also in such insulations thepenetration of the impregnant is hindered by the foil formed insulatingmaterial built up of flakes, being in itself impervious.

The impregnation difliculties which have hitherto existed have, as aconsequence, meant that for impregnation of insulations consisting of agreat number of insulating material layers, only impregnants with anexceptionally low viscosity have in practice been usable. In order tomake use of impregnants with higher viscosity possible it has beennecessary to build up the insulation step by step to the desiredthickness through a repeated procedure comprising wrapping on insulatingmaterial several times, a few layers at a time, with a subsequentimpregnation after each such wrapping.

It is an object of this invention to provide a process for producinginsulated conductors and coils having insulations comprising severalconsecutive layers of an insulating material and a resinous impregnant.Another object of the invention is to provide electrical conductors andcoils with solid voidless insulation. A still further object of theinvention is to provide conductors and coils with solid voidlessinsulations comprising mica and a solventless resinous impregnant.Another object of the invention is to provide a process for rapidly andeconomically impregnating an insulation applied to a conductor with aresinous impregnant. Other objects of the invention will appearhereinafter. For a better understanding of the invention, referenceshould be made to the following detailed description and drawings, inwhich:

FIGURE 1 is a schematic diagram of the process of this invention;

FIGURE 2 is a fragmentary perspective view of a coil being insulatedaccording tothe invention;

FIGURE 3 is a fragmentary perspective view of another coil insulatedaccording to the invention.

The invention relates to a process for producing insulated high voltageconductors and coils comprising wrapping a conductor or a bundle ofconductors with an insulating material which has been perforated inorder to radically facilitate the penetration of an impregnant,

and impregnating the wrapped conductor or coil with an impregnant. Theinvention refers to insulations with such insulating materials which inunperforated state are pervious only with difiiculty, or impervious forthe impregnant, such as, e.g., insulating materials containing layers ofmica flakes overlapping each other. It has been proved that thedielectric strength of an insulation built up of several layers of aninsulating material perforated in a suitable way, e.g., a perforatedmica foil, is not lower than the dielectric strength of a correspondinginsulation with unperforated insulating material. The use of aperforated insulating material makes it possible to use a lower pressureand a shorter time at the impregnation than is the case if correspondingunperforated insulating materials are used. Further the use ofimpregnants with a relatively high viscosity is made possible without,for this reason, it being necessary to build up insulation as beforestep by step to the desired thickness through a repeated procedurecomprising wrapping on a few layers of material several times, and asubsequent impregnation after each such wrapping. Instead, allinsulating material can be wrapped on in a single step to the desiredinsulation thickness before an impregnation of the insulation is made.

Referring now to FIGURE 1 of the drawing, there is a schematicillustration of'the steps of the process according to the invention.

The following are special examples of processes for producinginsulations described in connection with FIG- URE 2 and FIGURE 3.

Example 1 A plurality of individual conductors 1, in this case tenconductors, form the coil or bundle 2. The conductors 1 which have across section of 12 x 3 mm., are insulated from each other in such a waythat they are covered with glass fibre yarn 3 and impregnated with abinder, e.g., an epoxy resin such as Araldit 15 with Curing agent 15from Ciba A.G., Switzerland, the proportion by weight of the ingredientsof the resin being 100:30. The bundle of conductors 2 is wrapped inlayers with a perforated 0.09 mm. thick mica sheet 4. The mica sheetconsists of a self-supporting or self-carrying mica foil built up ofdimunitive mica flakes which overlap each other and which are heldtogether by molecular forces acting between the individual flakes. Theperforation has been performed by stamping out round holes 5 with 1 mm.diameter, whereby each square centimetre of foil area has three holesplaced by chance. When the whole bundle of conductors in the describedway, by wrapping in layers, has been provided with an insulationconsisting of thirty consecutive mica foil layers, and finally with onelayer of a 0.13 mm. thick and 30 mm. wide glass fibre tape withoutoverlap acting as a protective tape for the mica foil, the wrapped coilis placed in an impregnating tank, where moisture and other volatilesubstances are first removed at a pressure of about 0.1 mm. Hg and at atemperature of about 40 C. for about 2 hours. The glass fibre tape isnot shown in the figure. An impregnating resin consisting of 85 parts byweight of the epoxy resin Araldit F, 100 parts by Weight of Curing agent905, both from Ciba A.G., Switzerland, and 15 parts by weight of phenylglycidyl ether is then introduced into the impregnating tank at thementioned pressure, so that the coil is totally immersed in the resin.Thereafter a pressure of 5 kg./cm. is applied With nitrogen gas for 4hours. The temperature for impregnation is held at 40 C. The coil isthen withdrawn from the impregnant and wrapped with 30 mm. wide and 0.05mm. thick film tape of polyethylene terephthalate polymer with halfoverlap. The task of this tape, which is not shown in the figure, andwhich is practically impervious to the impregnant, is to prevent theimpregnant leaving the insulation before curing and to act as releaseagent during the forming of the insulation which takes place in a mouldat a temperature of about 160 C. and

at a pressure of 3 kg./cm. during a time for 4 to 6 hours. By thistreatment the liquid resin is cured and transformed into a solid resin.After the curing of the resin, the impervious tape is removed from thecoil. In FIGURE 2 the layers of mica foil wrapped around the bundle ofconductors are partially removed in order that the individual conductorsand their insulations will be visible.

The mentioned self-supporting mica sheet can be manufactured in awell-known way described in the US. Patent No. 2,549,880 and comprisingsplitting common mica by first heating it and then subjecting itsuccessively to the action of two solutions capable of reacting uponeach other so as to produce a disengagement of gas, thereafter placingthe split mica in water to form a pulp of mica flakes, which issubjected to a further treatment according to a method similar to thatused in the manufacture of paper. The mica sheet which is self-carrying,consists of incidentally arranged small mica flakes, overlapping eachother and being bonded to each other by molecular forces acting betweenthem.

Example 2 Referring to FIGURE 3, the high voltage coil 2 is composed ofa plurality of individual conductors 1 each provided with insulation 3.The insulated conductors have been moulded in a hot press and formedinto a bonded coil. The number of individual conductors is 20 and thecross section of each conductor 2.5 x 10 mm. The insulation of eachconductor consists of a glass fibre covering and of a polyester res-inof the same type as is used as impregnant for the insulation of the coiland described later. The coil is wrapped with 16 layers of an insulatingtape 6 with half overlap. The insulating tape is composed of common,large mica flakes attached to a backing of paper, 0.03 mm. thick, with abinder consisting of an unsaturated polyester resin of the same type asthe impregnant. The width of the tape is 25 mm. and the thickness 0.15mm. The tape is perforated by stamping out, on an average, 3 holes with1.4 mm. diameter per square centimetre of foil area. Over the insulatingtape a layer of glass fibre tape 7 of the same type as in Example 1 isapplied. The wrapped coil is subjected to resin impregnating processwherein the coil is first evacuated to remove all air and moisturetherefrom and then an unsaturated polyester resin is applied to theevacuated coil to ensure complete impregnation. The coil is immersed inthe resin at the pressure used at evacuating, 0.1 mm. Hg, and then apressure of 5 kg./cm. is applied. The temperature at impregnation isheld at 40 C. and the time for the pressure-period is one hour. Thepolyester resin used for the impregnation is manufactured in thefollowing way. Maleic anhydride, phthalic anhydride and propylene glycolin the proportion 2 moles of maleic anhydride, 1 mole of phthalicanhydride and 3.3 moles of propylene glycol are reacted in inertatmosphere under successive raising of the reaction temperature to 220C., after which the temperature is maintained at this level until theacid number of the reaction product is 30. The reaction product is thenadded to diallyl phthalate in such an amount that the product soobtained contains 30 percent by weight of diallyl phthalate. Further, 1percent by weight of benzoyl peroxide is added as a catalyst. After thecoil has been withdrawn from the impregnant it is wrapped with animpervious polyethylene terephthalate tape '8 of the same kind and inthe same way as described in Example 1. Thereafter the insulation isformed in a mould at a temperature of about C. and at a pressure of 3kg/cm. for one hour. The impregnant is thereby cured and transferredinto a solid resin. After the curing of the resin the impervious tape isremoved from the coil.

Example 3 Instead of the self-supporting mica sheet described in Example1 a 25 mm. wide and 0.09 mm. thick self- Example 4 Instead of the micatape described in Example 2, a mica sheet having the same backing,binder and perforation as the mentioned tape, being as broad as theconductor bundle to be insulated is used. The sheet is applied in 32layers. Materials and manufacturing conditions are otherwise the same asin Example 2.

Example 5 A 25 mm. Wide perforated insulating tape 6, composed of a 0.09mm. thick self-supporting mica foil built up of rdiminutive mica flakesoverlapping each other and being held together by molecular forces, andof a 0.0 25 mm. thick polyethylene terephthalate film being a backingfor the mica foil is used for insulating a coil in accordance withFIGURE 3. The perforation is made by stamping out holes with 1 mm.diameter, whereby each square centimetre of foil area has one holeplaced by chance. The binder in the tape is the polyester resindescribed in Example 2. Each conductor of the coil is provided with aninsulation consisting of a glass fibre covering and of the polyesterresin described in Example 2. The number of individual conductors is 30and the cross section of 613.611 conductor is 2.5 x mm. The coil iswrapped with 12 layers of the insulating tape with half overlap andfinally with glass fibre tape 7 as in Example 2. Air and moisture isremoved in an impregnating tank at 10 mm. Hg and 40 C. for 2 hours. Anunsaturated polyester resin is then introduced in the tank at thementioned pressure so that the coil is totally immersed in the resin.Thereafter a pressure of 5 kg./cm. is applied for one hour. Thetemperature is held at 40 C. The polyester resin is manufactured frommaleic anhydride, sebacic acid and diethylene glycol by using theingredients in the proportions 1 mole to 1 mole to 2.2 moles. Thereaction conditions are the same as mentioned in Example 2 but thereaction is not stopped until the acid number has reached the value 25.To the reaction product is added monostyrene in such an amount that thecontent of monostyrene in the product obtained is 35 percent by weight.Benzoyl peroxide is added in an amount corresponding to one percent ofthe impregnant. After the coil has been Withdrawn from the impregnant itis as before wrapped with a polyethylene terephthalate tape 8 and theinsulation is formed during curing of the liquid resin in a mould underthe same conditions as those mentioned in Example 2, after which theimpervious tape is removed from the coil.

Depending on the type of conductor to be insulated, the use to which itis to be put, and the electrical and mechanical requirements, dilferenttape or sheet formed insulating materials may be used, the number ofwrapped layers of insulating material may be varied, and the number ofholes per unit area and their diameter may be different. Also the choiceof impregnant and the conditions for impregnating and curing will bedifferent from case to case.

Besides insulations comprising consecutive layers of an insulatingmaterial containing mica layers, the invention comprises insulationsbuilt up of consecutive layers of other insulating materials perviousonly with difficulty or impervious, e.g., thermoplastic film, such aspolyethylene terephthalate film, polycarbonate film, cellulosetriacetate film, etc. Self-supporting mica tape and sheet may be used assuch, or with a suitable backing such as glass fibre threads andthermoplastic foils, mentioned earlier, as well as paper, glass fibrecloth, cotton cloth, etc. In mica tapes and sheets consisting of common,large mica flakes attached to a backing diiferent material such aspaper, glass fibre cloth, cotton cloth, asbestos cloth, asbestos paper,etc. may be used as backing. The mica flakes may be attached to thebacking, e.g., with a resinous binder such as a saturated or unsaturatedpolyester resin, an epoxy resin, a phenolic resin, a silicon resin, anasphalt resin, shellac, etc. The mica flakes may also be mechanicallyattached to the backing, e.g., they may be sewn on the backing withsewing thread.

The holes are distributed over the tape or sheet shaped insulatingmaterial and the sum of the areas of the holes within the piece of theinsulating material used suitably amounts to 0.2 to 10 percent,preferably 0.5 to 5 percent, of the area of the said piece of theinsulating material in unperforated state.

Besides the described completely polymerizable impregnants many othercommercially available unsaturated polyester resins and epoxy resins maybe used, as well as many different binders for the glass fibres coveringthe individual conductors of a coil. Although many unsaturated polyesterresins and epoxy resins suitable for the impregnation of electricalinsulations are well-known, some further examples of such resins may bementioned, so an impregnant consisting of 60 parts by Weight of areaction product of 3 moles of maleic anhydride, 1 mole of adipic acid,4.4 moles of ethylene glycol, manufactured according to the processdescribed earlier, and with an acid number of 30, and of 40 parts byweight of diallyl phthalate, and containing 0.75 percent of benzoylperoxide may be used as Well as an impregnant consisting of 70 parts byweight of a reaction product of 1 mole of fumaric acid, 1 mole ofphthalic acid and 2.2 moles of propylene glycol reacted to an acidnumber of 25, and of 30 parts by weight of monostyrene, and containing0.5 percent of benzoyl peroxide. Among suitable epoxy resins may bementioned a product consisting of parts by weight of the epoxy resin,Epon 820 from Shell Chemical Co. and of 65 parts by weight ofhexahydrophthalic anhydride and a product consisting of 100 parts byweight of the epoxy resin Dow 331 from Dow Chemical Co. and of 65 partsby weight of tetrahydrophthalic anhydride. As impregnants furtherdifferent types of other resins, both natural resins such as, e.g.,asphalt, and synthetic resins usually used as impregnants, may be usedwith or without added solvents.

By using asphalt without a solvent as an impregnant, the impregnation isperformed at a temperature above the melting point of the asphalt, andthe insulation may suitably be formed in a mould in which the asphalt istransferred to a solid state.

It is obvious to those skilled in the art that a coil to be insulatedneed not, as the coil described in Examples 1 to 5, consist of a bundleof individual conductors, but may just as well consist of only oneconductor.

We claim:

1. An insulated high voltage coil comprising at least one conductingmember, at least twelve layers of an insulating tape comprising a micatape provided With perforations and having a woven glass fibre backinglayer, wrapped around the conductor, the sum of the areas of theperforations being from 0.2 to 10 percent of the total area of the micatape, and a solid resin derived from a liquid resinous compositionapplied to the electrical conductor and the wrappings thereon toimpregnate the wrapping and to bond the whole into a solid member.

2. An insulated high voltage coil comprising, in combination, anelectrical conductor comprising at least one conducting member, at leasttwelve layers of an insulating tape comprising a self-supporting micatape built up of diminutive mica flakes which overlap each other andwhich are held together by molecular forces acting between them, andhaving a woven glass fibre backing layer, wrapped around the conductor,the mica tape being provided with perforations the sum of the areas ofwhich is from 0.2 to 10 percent of the total area of the mica tape, anda solid resin derived from a liquid resinous composition applied to theelectrical conductor and the wrappings thereon to impregnate thewrappings and to bond the whole into a solid member.

3. An insulated high voltage coil comprising, in combination, anelectrical conductor comprising at least one conducting member, at leasttwelve layers of an insulating tape comprising a mica layer havingoverlapping mica flakes and having a woven glass fibre backing layer,

Wrapped around the conductor, the mica layer being provided withperforations the sum of the areas of which is from 0.2 to 10 percent ofthe total area of the mica tape, and a solid resin derived from a liquidresinous composition applied to the electrical conductor and thewrappings thereon to impregnate the wrappings and to bond the Whole intoa solid member.

References Cited in the file of this patent UNITED STATES PATENTS1,730,740 Morrison Oct. 8, 1929 2,656,290 Berberich et a1. Oct. 20, 1953

1. AN INSULATED HIGH VOLTAGE COIL COMPRISING AT LEAST ONE CONDUCTINGMEMBER, AT LEAST TWELVE LAYERS OF AN INSULATING TAPE COMPRISING A MICATAPE PROVIDED WITH PERFORATIONS AND HAVING A WOVEN GLASS FIBRE BACKINGLAYER, WRAPPED AROUND THE CONDUCTOR, THE SUM OF THE AREAS OF THEPERFORATIONS BEING FROM 0.2 TO 10 PERCENT OF THE TOTAL AREA OF THE MICATAPE, AND A SOLID RESIN DERIVED FROM A LIQUID RESINOUS COMPOSITIONAPPLIED TO THE ELECTRICAL CON-